In order to protect the pressure responsive element of a pressure responsive instrument, such as a pressure gauge, from damage and contamination, as for example, when measuring fluids subject to freezing or corrosive or abrasive fluids, it is a common practice to impose an isolation device between the element of the pressure responsive instrument and the fluid undergoing measurement. The isolation device serves to physically separate the pressure responsive element of the instrument from the fluid being measured. Typically, isolation devices comprise pistons or diaphragms that are responsive to pressure fluctuations of the fluid undergoing measurement. Such isolation devices are described and claimed in U.S. Pat. Nos. 2,207,807 issued to Hopkins on July 16, 1940 and entitled "Pressure Responsive Indicating Apparatus"; 2,881,617 issued to Deis on Apr. 14, 1959 and entitled "Pressure Measuring Instrument"; and 3,718,046 issued to McJones on Feb. 27, 1973 and entitled "Pressure Gauge Isolator".
Such prior art isolation devices include complex structure tending to decrease the performance characteristics and reliability of instruments. Furthermore, such prior art devices have proven to be unsatisfactory when the fluid being measured undergoes wide temperature and pressure variations.
A need has thus arisen for an isolation device interposed between the pressure responsive element of a pressure responsive instrument and the fluid undergoing measurement that is simple in operation and reliable for accurately transmitting pressure from the source to the pressure responsive element. Such an isolation device should also function to isolate the pressure responsive mechanism of the instrument from the fluid undergoing measurement to prevent contamination.